This disclosure relates to insulated metal substrates having thermoplastic films for dielectric layers, and methods of manufacture thereof.
An insulated metal substrate (IMS) includes metal traces, a dielectric layer, and a metal substrate. Polymer films have been used as the dielectric layer in IMS laminates, but polymer films are generally poor thermal conductors. Typical thermal conductivities for polymer films can be about 0.2 to about 0.25 watts per meter-Kelvin (W/mK). To make the polymer films more thermally conductive, conductive particles, such as ceramic fillers, are added. The filled polymer films, however, have lower breakdown voltages per mil thickness than their unfilled counterparts. Breakdown voltage is a characteristic of an insulator that defines the maximum voltage difference that can be applied across the material before the insulator collapses and conducts. As such, a thicker polymer film, in some cases greater than 75 microns, is needed to satisfy breakdown voltage requirements for the IMS laminate. Moreover, the use of thermally conductive particles can be expensive, and the subsequent dielectric layer must also be thick enough to ensure it is pin-hole free. Because the filled thermoplastic dielectric layers are so thick, they add thermal resistance to the design. Examples of IMS laminates using filled polymer films for dielectric layers are Bergquist Insulated Metal Substrate® thermal boards and Denka HITT® plate boards. The dielectric layer of each of these IMS laminates are epoxy-glass or epoxy-alumina based and have thermal resistance values of about 0.05 Kelvin square inches per watt (K in2/W).
Some IMS laminates utilize unfilled polymer films, such as polyimide (PI) film. These unfilled polymer films, however, still have to be thick enough to satisfy breakdown voltage requirements, and therefore, have higher thermal resistance values. For example, PI film manufactured by Kaneka, such as Pixeo FC-622, can have a thickness of 14 microns and a thermal resistance of 0.10 K in2/W. Still not an optimal thermal resistance value for an IMS laminate.
In another attempt to circumvent the problem, dielectric layers have been made thermally conductive through the addition of a thin anodization layer on top of an aluminum heat spreader layer. An example of such IMS laminates are Anotherm® boards produced by TT Electronics®. The use of anodization as the dielectric layer attempts to overcome some of the issues associated with thick, filled thermoplastic dielectric layers, but forces the use of aluminum as its heat spreader layer, since copper cannot be anodized. Since the thermal conductivity of aluminum is significantly less than that of copper, this can be another thermal disadvantage. Additional limitations of this approach arise from the lack of flexibility to fabricate bent or non-planar circuit structures, and the fact that the dielectric material covers the entire surface of the heat spreader layer.
Moreover, all of the foregoing approaches, can suffer soldering difficulties, since the same heat dissipation properties that are useful during the operation of the printed circuit board and components, inhibit an assembly process that requires point sources of heat for soldering (such as hot bar bonding, for example).
Accordingly, there remains a need in the art for improved IMS laminates utilizing a thin thermoplastic film which does not so significantly increase the thermal resistance and lower the breakdown voltage of the dielectric layer.